There is a need for methods to reliably store gases that do not require high pressures to minimize the cost of compression and the cost of strong bulky tanks and to increase safety. Furthermore, for mobile applications, these storage systems should be light, small and rugged. The background of this invention can be better illustrated if we use natural gas and a carbon adsorbent as an example but it should be understood that the invention applies to many other gases and porous substrates.
Currently, natural gas can be compressed as a liquid at low temperatures as liquefied natural gas (LNG), which contains about 72% of the energy of gasoline. The major limitation of using LNG is that the liquefaction procedure is costly, it requires expensive treatment stations, it has the inherent danger of developing high pressures if it warms up and, therefore, although this approach is used industrially and even in trucks, it is not easily adaptable to small passenger vehicles.
Natural gas (NG) can also be stored by compressing it up to 3,000 psi at room temperature, forming compressed natural gas (CNG); as a supercritical gas, its density with respect to NG increases by about 230 times, and it contains about 28% of the energy of gasoline. High pressure CNG can already provide an acceptable performance for vehicles. The major drawback of this approach is that it needs expensive pumping stations that would require a major change in the infrastructure of the USA and large bulky storage tanks that occupy a significant volume of the trunk, or there is a need to redesign the car to accommodate the volume of the tank.
To circumvent all these limitations, the adsorption of NG in sorbents with high surface areas has been investigated. The quantity of methane adsorbed as a function of surface area has been measured for various high-surface-area sorbents including silica gel, carbon, molecular sieves, and metal-organic framework (MOF) sorbents. As much as 0.2 gCH4/g sorbent can be adsorbed at 35 bar at 298 K. At a pressure of 35 bar, the effect of surface area is more important than the composition of the sorbent. The bulk densities of silica-based materials range from 0.2 to 0.5 g/cm3 whereas for carbon based sorbents, it can range from 0.3 to 1.1 g/cm3. For the current applications, the mass of natural gas stored per unit volume of the sorbent is a critical parameter, as the container space is limited.
From the results available in the literature, it can be concluded that the goal of energy density will be difficult to reach using silica-gel, a molecular sieve, or MOF-based sorbents because they cannot be obtained as rugged solids at the surface areas needed to absorb enough methane. Furthermore, both zeolites and MOF are weak and unstable in water vapor and in the presence of impurities such as H2S, CO2, etc.
Carbon sorbents can be obtained industrially with high surface areas (e.g., >1500 m2/g) and high densities (e.g., >1 kg/l) and are chemically stable under the conditions of absorption.
Relevant art: U.S. Pat. Nos. 6,743,278 and 7,494,530.